Cancer is the second leading cause of death in the United States, killing more people than the next five causes combined including chronic respiratory disease, Alzheimer's disease and diabetes. While extraordinary strides have been made in the detection, prevention and treatment of cancer, there remains an urgent need, especially in advanced cases, to produce therapies that not only halt tumor progression but effectively eliminate all tumor cells. One approach is adoptive T cell immunotherapy (5-7). This method requires the harvesting of the patient's T cells, engineering of these cells with a chimeric antigen receptor (CAR) that recognizes a tumor antigen, and subsequent re-introduction of the modified cells to the patient. The re-programmed T cells then directly target antigen-expressing tumor cells, bypassing the requirement for MHC peptide, and elicit a powerful but localized immune response. This method of treatment (8) has produced some positive results in early clinical trials for a handful, but not for all patients. There is a need in the art to better understand CAR T cell therapy's success and failure. For example, there is a need in the art for the ability to characterize the density of the CARs on the transformed cells, to track administered CAR T cells at any point during therapy and correlate this distribution to therapeutic outcomes, to rapidly functionalize CAR T cells, monitoring the number, location and viability of the transplanted CAR T cells in situ and to selectively eliminate CAR T cells if necessary. Meaningful correlations would aid clinicians in determining the best treatment options and give researchers important clues to modify and improve this therapeutic approach. Provided herein are compositions and methods addressing these and other needs in the art.